The present invention relates to a communication method and wireless network system for exchanging data in the form of digital signals.
One known data communication system with wireless transmission technology is the accessing method specified in IEEE 802.11. Such a wireless network system is schematically explained referring to FIG. 15.
In the FIG. 15 system, upon finding that a particular transmission channel is active, control station 1 (an accessing point) transmits a control signal of a constant period, called a beacon. Control station 1 controls a group of communication stations 2, 3, 4, 5, and 6 for exchanging packets of data. The communication stations 2-6 are notified by the beacon that the channel is active. Each communication station then registers itself as a subscriber for data exchange under control station 1. Then, the communication station is qualified to participate in packet data communication on the transmission channel.
The exchanged data may usually be classified into two types, asynchronous data (common computer oriented data), which are less susceptible to transmission delay and jitter, and synchronous data (video data or the like), which are highly susceptible to transmission delay and jitter. It is assumed that communication stations 2, 3, 4, and 5 handle both types of data while communication station 6 handles only synchronous data.
A common contention mode communication technique will now be explained. Before starting communication in a contention mode, the communication station monitors or senses the carrier transmitted over the channel within a predetermined period of a distributed interframe space (DIFS). Only when the carrier is not found and the transmission channel is free, can the transmission of packets be started. This is known as carrier sense multiple access (CSMA).
When the carrier is present and the transmission channel is busy, the transmission starts just after the expiration of a random duration of time (known as a back-off interval) for accounting DIFS and avoiding interference with other communication stations. This may not ensure avoidance of collision of transmission signals but it minimizes the possibility of collision. This is known as collision avoidance (CA). In Ethernet of cable communications, a collision can be detected by the communication station. This is known as collision detection (CD). In the wireless communication system, the received signal is however offset by the transmitting output thus permitting no detection of the collision.
For detection of collision in a wireless communication system, the communication station at the receiving side transmits an acknowledgement in response to the arrival of packets transmitted by the transmitting station at the other side. This allows the transmitting station to have knowledge of the arrival of the packets at the receiving station. If no acknowledgement is received, the transmitting station determines a collision or transmission error has occurred and repeats the transmission of the packets.
What is explained above is known as carrier sense multiple access with collision avoidance (CSMA/CA), which is a type of accessing method for contention mode communication. This method is basically arranged so that plural communication stations compete with each other for occupancy of an available transmission channel. Hence, during the transmission of packets, discrepancies (jitters) may occur, and it will thus be impossible to predict the precise time of arrival of packets at the receiver station.
The technique of contention-free mode communications specified as an option in IEEE 802.11 will now be explained. The contention-free mode communication method is designed such that, during a particular period of time (the duration of contention-free mode communication) determined by the timing of transmission of a beacon from the control station, the contention mode communication between communication stations is canceled and the control station is operated to determine and give transmission priority to one of the communication stations through polling the communication stations. During this period, no collision of signals may consequently occur between the communication stations.
The contention-free mode communication method is suitable for transmission of data throughout a given duration, e.g. audio and video (AV) data, which are highly susceptible to jitters, and thus are particularly suited for broadcasting. While carrying out the subscriber registration procedure, the communication stations announce to the control station whether or not they use the contention-free mode.
Using information from the communication stations, the control station produces a list of the communication stations to be polled, namely a polling list. During the contention-free mode communication, the communication stations included in the list are polled one after another. For example, when communication station 2 intends to send a synchronous transmission to station 3, it transmits the synchronous transmission data packets, which are finally destined for station 3, to control station 1 upon being polled by control station 1.
The packets are held in control station 1 and transferred to station 3 by control station 1 together with its polling message at the time of polling of station 3. During the contention-free mode communication, each transmitting action is governed by control station 1 thus eliminating the collision of signals.
However, in the method of communication through a polling action of the control station during the contention-free mode operation, data which can be received directly by the destination station has to always be transferred via the control station. This decreases transmission efficiency to about xc2xd as compared with common direct transmission between two communication stations.
Also, in this method, the timing of starting the transmission depends largely on the size of data transmitted to another station at the preceding polling operation and the size of data to be transmitted in response to the current polling operation. If the timing is altered, jitters may accordingly occur even in this method.
Moreover, the synchronous transmission of bulky data such as AV data requires an extended length of time for connection to the transmission channel and occupies a large portion of the transmission signal band, resulting in shortage of the band range, particularly during transmission between plural communication stations.
In common wireless communications, two or more transmitting operations at one time may be permitted by adjusting the transmission power when the location of the stations, the number of obstructions, and the conditions for aerial transmission are not hostile. However, the contention-free mode communication method fails to address such simultaneous transmitting operations.
According to the method and apparatus of the present invention, a particular period involving no polling action, called time-slots, is assigned by the control station in the contention-free mode communication period. Throughout the particular period, synchronous communications can be performed directly between the communication stations. Prior to the direct communications, the communication station of interest conducts a call-connection procedure followed by a subscriber registration procedure for being under the control of the control station, and the communication station is then admitted by the control station for use of allotted time-slots.
A step of transmitting a test packet is added to the call-connection procedure, and the transmitting communication station examines whether or not it can communicate directly with the receiving station. When so, the two stations can communicate with each other without being relayed at the control station. The problem of contention-free mode communication, i.e., that the transmission rate decreases to xc2xd as compared with the direct transmission, can consequently be eliminated.
For allowing plural call-connections to be initiated at the same time-slot, the communication station includes:
(1) means for gaining data about the wireless signal reception state(such as quality), for example, the power level of the received wireless signal or the transmission error rate of the wireless signal;
(2) means for controlling the power level of signal transmission; and
(3) means for, upon receiving a test packet or a test response packet, including in the test packet or the test response packet data describing the measured reception power level or the error rate of the previous packet received at the communication station.
The above arrangement permits the communication station to monitor the signal reception state at itself and at another communication station during transmitting and receiving the test packet and the test response packet before the communications. The communication station lowers the signal transmission power to such a level that the signal reception state stays above a threshold, thus minimizing the interruption of other communication stations with its transmitting signal. Consequently, the possibility for enabling synchronous communications between any two stations is significantly increased.
Alternatively, even when another communication station sets up another call-connection oriented in a different direction, it is more likely that the communication station can communicate with that destination station with the use of a directional antenna. This will further encourage simultaneous synchronous communications, in combination with wireless transmission level control.
However, using this method may cause another problem. When the communication station moves, it is unlikely that its signal transmission level and optimum antenna directivity control will be maintained. For compensation, the present invention provides a re-test procedure, in which the communication station transmits the test packet again. Any setting change triggered by movement of the communication station can consequently be detected and controlled dynamically to maintain optimum communication conditions.
If it is found from the re-test that relevant control parameters, such as the transmission level and the antenna directivity, are substantially identical to those measured in the original test, the conclusion is that the communication station has not moved but the communications have been interrupted by other causes. To deal with that, a packet requesting a change of time-slots is transmitted from the communication station to the control station. The control station controls the call-connection initiated in each time-slot with a call-connection table. The call-connection which cannot be performed among synchronous simultenous communications can be identified and shifted to another free time-slot by the control station, hence allowing the continuous operation of wireless communications.